If you order them to 'stud with <metal>', that's exactly what they'll do, as long as the metal you're studding with is not yet present in the item - you can't stud a steel breastplate with steel, but you can stud it with gold, silver and platinum. Metalcrafters will decorate absolutely any finished item they can get their fingers on, including stuff jewellers won't touch: weapons, armour and bins.
Have you actually tested this out with linked stockpiles or something? I have never got it to work with metal or gems.
Granted, I never tried a linked stockpile in the new versions.....
I know this is off topic, but someone asked.
Yes, with the new stockpile links it's pretty easy to do. You link a bar stockpile with a forge (and to stud it uses no fuel so it doesn't matter the fuel source). You then link a weapon or armor stockpile to that same forge. If you do this, you will be able to only stud your adamant or steel gear and not have a crafter studding every single finished good in the fort which used to be the problem unless you burrowed.
I did this with a legendary armorer and a legendary metalcrafter and got dozens of Masterful adamantine weapons and armor studded with superior and masterful quality gold, bronze, aluminum, platinum, menacing with spikes of copper and with masterful images of XX in silver, pewter, &c.
Then, after that fortress fell, I took my adventurer to it, stole all the adamantine stuff and brought it to a abandoned embark. Then I reclaimed it and immediately had candy for everyone. I don't usually do that, but after spending all the time and effort to get a legendary armorer, weaponsmith, and metal crafter to work each piece of adamant, I couldn't help myself. I was more attached to those three dwarves than any other dwarf I've ever had. I figured it would be an insult to their hard work to let their masterpieces sit there forever.
Tun Othilisat, Sarvesh Cugancin, and Nish Uristinol, your work has not been in vain.